1. Field of the Invention
The present invention concerns a method for controlling a print head operating according to the thermotransfer principle with a number of printing elements, of the type wherein an energy quantity is fed to a printing element in a feed step in order to transfer ink from an ink carrier device associated with the print head to a substrate associated with the ink carrier device to generate an image point of a barcode. The invention furthermore concerns a printer that is suitable for implementation of the inventive method.
2. Description of the Prior Art
The machine-readability of barcodes, particularly two-dimensional barcodes, depends heavily on the print quality. This is particularly true for two-dimensional barcodes with very small module sizes. For example, for the franking imprints accepted by the Canadian Post a two-dimensional barcode is required that is composed of 48×48 modules (printed or non-printed rectangular fields) on an area of 1 inch×1 inch, such that an edge length of the respective module of approximately 0.5 mm results.
Basic criteria for the print quality (and therewith the machine-readability) of such a barcode are a uniform size of the modules in both directions and a uniform coverage over the area of the entire barcode.
In order to obtain a qualitatively high-grade barcode in such thermotransfer printers as they are known from DE 40 26 896 A1, for example, the respective printing elements of the print head must be supplied with a relatively precisely dosed energy amount for each image point to be printed in order to reliably melt the ink particles in the desired quantity to achieve the desired spatial expansion of the carrier material of the ink ribbon. Depending on the current temperature of the respective printing element, more or less energy must be supplied in order to achieve an optimal melting temperature.
Furthermore, from DE 10 2004 063 756 A1 it is known in connection with franking imprints to use different printing parameter sets for different regions of a franking imprint with different print image types (clear text/graphics, one-dimensional barcode, two-dimensional barcode) in order to satisfy the different requirements of these print image types.
As in the printer known from DE 40 26 896 A1, the calculation of the energy quantity to be introduced into the appertaining printing element for the respective image point to be printed is undertaken for the region of a barcode to be printed dependent on the total energy input into the printing element, this energy input occurring as a result of the heat conduction from adjacent printing elements that were previously activated for printing, as well as the residual energy that still exists due to previous printings by the current printing element.
A relatively precise control of the printing elements is thereby possible, but a disadvantage is that a relatively complicated calculation is required for each image point to be printed, which reduces the processing speed for a print image and thus the throughput, of the printer also is reduced. In known printers this can be counteracted by providing more processing capability, thus requiring a more complicated and more expensive processor.